This invention is in the field of optical particle counters. This invention relates generally to calibration verification systems and methods for verifying the calibration status and performance of optical particle counters.
A large portion of the micro-contamination industry is reliant on the use of optical particle counters, such as are described in a large number of U.S. patents, including U.S. Pat. Nos. 3,851,169, 4,348,111, 4,957,363, 5,085,500, 5,121,988, 5,467,188, 5,642,193, 5,864,399, 5,920,388, 5,946,092, and 7,053,783. U.S. Pat. Nos. 4,728,190, 6,859,277, and 7,030,980, also disclose optical particle counters and are hereby incorporated by reference in their entirety. Aerosol optical particle counters are used to measure air-born particle contamination in clean-rooms and clean zones. Liquid particle counters are often used to optically measure particulate contamination in the water treatment and chemical processing industries.
Optical particle counters for these applications generally have one year calibration cycles. International standards such as JIS B 9921: Light Scattering Automatic Particle Counter, ASTM F328-98: Standard Practice for Calibrating an Airborne Particle Counter Using Monodiperse Spherical Particles, and ISO/FDIS 21501-4: Determination of particle size distribution—Single particle light interaction methods—Part 4: Light scattering airborne particle counter for clean spaces are available that detail the calibration requirements for optical particle counters. 
The calibration process for an optical particle counter is complicated and usually requires trained representatives from the particle counter manufacturer to perform the calibration. The calibration process is centered on the use of certified particle size standards. In the United States, these standards are water suspended polystyrene spheres, traceable to the National Institute of Standards and Technology (NIST). A typical particle generation system for providing calibration standard particles to an aerosol optical particle counter is exemplified as a Model PG-100 Particle Generator 100, as shown in FIG. 1.
In reference to FIG. 1, air is pulled into the system at position 101 by means of a pump 102. This air is filtered 103 to remove any particles in the flow. As the calibration standard particles are water suspended, they must be aerosolized by a particle generator for detection by an aerosol optical particle counter. The water and particle mixture is placed into a nebulizer 104 where it is aerosolized by the nebulizer with a stream of pressurized air produced by the particle generator pump. Valve 105 regulates the flow rate through the nebulizer 104. The particle generator pump 102 is typically large, heavy, and power consuming, generally requiring AC power. The air and water mist (water droplets that contain polystyrene spheres) from the nebulizer is then combined with any residual airflow from the particle generator pump 102 through bypass valve 106 and passed through one or more drying chambers 107 and 108 in order to allow the evaporation of the water droplets. Once the water droplets have evaporated, only the polystyrene spheres remain in the particle generator air flow. As shown in FIG. 2, the particle air flow is then combined with a larger filtered air flow provided by a filter-tee assembly 109 in order to provide the particle counter (exemplified in FIG. 2 as a LASAIR Model particle counter 110) with the full amount of its required air flow.
In this manner, mono-dispersed particles of known sizes are used to calibrate each corresponding particle channel of the particle counter. For example; a 1.0 μm particle is used to calibrate a 1.0 μm channel. This ensures that each particle channel of the unit under test sizes particles accurately.
In addition to the testing described above, it is also generally required to inter-compare the test instrument with a reference particle counter. This is done, for example, to ensure the test instrument achieves 50% counting efficiency at its stated first channel particle size, and 100% counting efficiency at 1.5 to 2.0 times its stated first channel particle size. Comparison testing requires an entirely different flow system in addition to the one shown in FIG. 2 in order to ensure a homogenous mixture of filtered air and particles is delivered to both the unit under test and the reference instrument.
It is generally also required to measure and confirm the flow rate of the unit under test with a NIST traceable flow meter, as well as to perform a zero-count (false count rate) test. For some application, the instrument is generally required to demonstrate an achievable false count rate of less than one count in five minutes with a 95% upper confidence limit. This test is very time consuming and may require an extended total sampling time of over an hour.
A full optical particle counter calibration is complicated and, thus, generally must be performed by a trained representative from the particle counter manufacturer. A full calibration typically requires a large amount of test equipment which is not portable. Generally, the optical particle counter under test must be brought to the calibration equipment location.
For cost reasons and ease of implementation, particle counter users generally limit calibration to a one year calibration cycle, as typically recommended by particle counter manufacturers. The industry range for aerosol particle counter calibrations is $300 to $1200, per instrument, per calibration. Particle counter users must assume that the particle counter will maintain proper calibration throughout the one year calibration cycle, although this is sometimes not the case.
Aerosol particle counter users are segmented into a number of industries. For example, the semiconductor and pharmaceutical industries are two industries for which particle measuring plays a significant role. Semiconductor users generally monitor aerosol contamination in order to improve or maintain wafer yield levels. If an aerosol optical particle counter drifts out of calibration in one of these clean areas, the particle counter may over- or under-count the particle level in the clean area. If the particle counter is under-counting, the clean area may be dirtier than the user believes. In a worst case scenario, the user may experience a drop off in wafer yield due to this undetected particle contamination. While the drop off in yield is undesirable, the user is at least given real time feed-back from quality control monitoring of the wafer yield, and will have some stimulation to investigate a possible problem within that specific clean area. Ultimately, the out of calibration particle counter would be revealed as the reason for the drop off in wafer yield.
An out of calibration particle counter also presents significant problems to a pharmaceutical user. Pharmaceutical users must monitor clean areas where pharmaceuticals are handled or processed. In the United States, this monitoring is mandated by the Food and Drug Administration (FDA). The process areas must be maintained to a specified cleanliness level established for certain pharmaceutical products. If a particle counter is under-counting, the clean process area may be dirtier than the user believes. The user has no means of detecting the out of calibration particle counter, as there is no real time feed-back of any process that would indicate a problem. The user may continue to process pharmaceuticals in the suspect clean area for the remainder of the annual calibration cycle of the particle counter, before finally being informed that the particle counter was out of calibration upon its next scheduled calibration.
As liquid particle counters are often used to optically measure particulate contamination in purified water and chemical streams, when a liquid particle counter is under-counting, the water or chemical streams may include particulate levels higher than a user believes. For example, if a liquid source includes particle levels higher than expected, this may result in an end product, of which this liquid which is a component, having contamination levels higher than expected. As above, this may pose a significant problem, for example if the end product is a pharmaceutical composition. Alternatively, if the liquid is utilized, for example, as a rinse, wash or solvent during the processing of a semiconductor device, particle contamination of the semiconductor device may result, resulting in decreased semiconductor device yield.
Once the particle counter is defined as out of calibration, the status of the clean area that it monitored for that entire calibration cycle (typically a year) is in question. If it is determined that the particle counter sufficiently under-counted so as to place the actual clean area it monitored above the allowed FDA specified contamination limit, all product produced in that area for the entire year becomes suspect. The user may be forced to recall the entire year's pharmaceutical product produced in the suspect area. This is a disaster that could cost the pharmaceutical user millions of dollars in lost product.
Calibration systems and methods for evaluating the calibration status of optical particle counters can be found in U.S. Pat. Nos. 4,360,270, 4,434,647, and 5,684,585. U.S. Pat. No. 4,360,270 discloses passing thin, translucent fibers through the laser of an optical particle counter to give a fixed, repeatable signal useful for determination of calibration status of the optical particle counter. Similarly, U.S. Pat. No. 4,434,647 discloses passing a probe with an opaque, precisely sized circular spot through the laser of an optical particle counter. U.S. Pat. No. 5,684,585 discloses modulation of the laser beam intensity to simulate the detection of particles of a known size. These particle-type events may be compared to those previously determined when the particle counter was known to be properly calibrated to verify if the particle counter remains properly calibrated. The primary drawback to these and similar methods and systems is that they do not employ actual particles for testing the optical particle counter. Use of particles similar to those used during calibration is beneficial for ensuring that the calibration verification can be relied upon as an accurate measure of the continued calibration (or identified mis-calibration) of the particle counter.
Other methods are known for the calibration verification of a particle counter, such as described in U.S. Pat. No. 5,747,667. This patent discloses passing a known number of particles through a particle counter and comparing with the actual number of particles detected. Although this method is useful and uses standardized particles, it appears to only consider counting accuracy and is practical only for particle counters which measure particles suspended in a liquid. Optical particle counters may become mis-calibrated for a number of reasons including drift in the detection electronics; such a mis-calibrated particle counter may still properly count the total number of particles but may misidentify the real sizes of particles.